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A bound state consisting of an electron and a positron. There are two types of positronium: orthopositronium, in which the spins of the two constituents are parallel, and parapositronium, in which the spins are anti-parallel. Both forms have brief existences, with orthopositronium decaying into three photons in about 1.5 × 10−7 s and parapositronium decaying into two photons in the even shorter time of about 10−10 s. Positronium has a hydrogen-like spectrum, but with different values of the frequencies since a positron is much lighter than a proton.

Subjects: Physics.

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